Related Field
The invention relates to a measuring device and a method for measuring parameters of a body tissue using an elongated probe for insertion into the body tissue as set forth in the preamble of claim 1, in particular a measuring device for measuring cerebral parameters by means of an invasive measuring procedure.
Description of Related Art
Various invasive procedures for cerebral diagnostics and therapy are known in which different cerebral parameters are measured, such as cerebral hemodynamics. Measured are, for example, parameters relating to the concentration of deoxygenated and oxygenated hemoglobin, the mean transit time of an indicator, the cerebral blood volume, the cerebral blood flow or the tissue oxygen index. Among the methods of this kind are e.g. the near-infrared spectroscopy (NIRS) or the pulse oximetry. For capturing cerebral parameters subdural measurements, measurement on the brain surface, ventricular measurements or intraparenchymal measurements may be carried out.
Known from EP 1301119 B1 is e.g. a subdural probe and a device for measuring cerebral hemodynamics and oxygen saturation. For this purpose the probe comprises a first optical guide, which guides light to the distal end of the probe and thus to the head and into the brain of a patient, and a second optical guide, which guides light from the distal end to a proximally situated processing device. The optical guides are disposed substantially parallel to one another. Provided at the distal end of the optical guides are optical units which deflect light guided by the optical guides out of the transmission direction along the guide by an angle of e.g. 90°. The light is thereby redirected e.g. by the optical units from a direction parallel to the dura vertically into the brain tissue. The optical guides and the optical units form emitting and receiving optodes, and are surrounded by a covering of elongated shape. The spacing of the optical units determines the implantation depth, to which light is able to penetrate into the tissue and be reflected or scattered back. Light reflected or scattered by the brain tissue is directed to the second optical unit, and is thereby fed to the optical guide from a direction approaching perpendicular to the transmission direction, and is guided in transmission direction to the processing device. With such a probe the light exits laterally in radial direction, and reflected and scattered light is also captured again in radial direction on the same side. A significant portion of the light can thereby reach the receiving optode directly on the side of the probe along the surface thereof without penetrating the tissue to be measured. The measurement signal can thereby be interfered with or distorted. Furthermore localization of the measured tissue region is difficult in the case of a measurement on the side of the probe.
Known from U.S. Pat. No. 5,579,774 is moreover a device for measuring the blood flow in cerebral areas, which device can be inserted at a right angle into cerebral tissue. The device has a plurality of sensors along an elongated measuring probe for carrying out a Laser Doppler Flowmetry (LDF) measurement. The sensors are provided at openings along the length and the circumference of the measuring probe and at its tip. The sensors are connected to a light source by means of optical guides which have at the end a reflecting surface that directs the light in a way substantially perpendicular to the measuring probe into the surrounding tissue. The spacing of the sensors must be large to such an extent that light which is emitted from a sensor and is reflected or scattered by the tissue is not able to be captured by another sensor. With this measuring device the emitted and the captured light are thus emitted and captured at the same opening in the measuring probe. The measurable area within the cerebral tissue is thereby limited in an extreme way, and the measurement can be distorted by the emitting and receiving opening being situated together.
U.S. Pat. No. 4,986,671 shows an optical measuring probe for measuring pressure, temperature and flow rate in blood vessels. The measuring probe can be provided on a catheter, which is inserted into the blood vessels. The measuring probe has an optical transmission medium which guides light of different wavelengths to the tip of the probe for measuring different parameters. Provided at the tip of the probe is an elastic optical element which forms an inwardly convex surface that is coated with different materials. Light which is guided by the optical transmission medium to the elastic surface of the optical element is reflected there. The parameters to be measured can be determined from the light that is deflected on the deformed elastic surface, the deformation being characteristic for the parameter values to be measured. With this measuring method no light exits from the measuring probe into the surrounding tissue, so that the measuring range of the probe in the surrounding region is very small. Furthermore some relevant parameters, such as oxygen content, cannot be determined with such a measuring probe.